Since nonaqueous electrolyte secondary batteries represented as lithium ion secondary batteries have high energy density, the nonaqueous electrolyte secondary batteries are used as power sources for mobile communication and portable information terminals. In recent years, the nonaqueous electrolyte secondary batteries have been started to be practical for use in vehicles, and the nonaqueous electrolyte secondary battery market has rapidly expanded. Accordingly, in order to pursue a further reduction in size and weight of equipment, there has been a demand for performance improvement for achieving a further reduction in size and weight of batteries occupying a large volume in the equipment.
Currently, negative electrode active materials used in the secondary batteries (hereinafter, referred to as “active materials” in some cases) are mainly graphite-based carbon materials. The graphite-based carbon materials are key materials influencing the performance of the batteries. However, an amount of lithium which can be reversibly intercalated and deintercalated in the graphite-based carbon material is limited to one lithium atom per 6 carbon atoms. A theoretical charging/discharging limit capacity of the carbon material calculated from the limit value is 372 mAh/g in terms of electric capacity. Since the current secondary batteries have been used at a level close to the limit capacity, it is difficult to expect a remarkable performance improvement in the future.
Under the circumstances, searches for materials other than carbon are being conducted, for example, materials which are alloys or inorganic compounds and have an electric capacity of much higher than 372 mAh/g. Among them, particularly, in crystalline oxide materials containing tin and/or silicon or amorphous oxide materials, materials exhibiting a discharge capacity close to 1,000 mAh/g have been found (for example, refer to Patent Documents 1 and 2).
However, the above-described high capacity active materials undergo larger volume fluctuations, caused by lithium intercalation and deintercalation, than the graphite-based carbon materials in the related art. Thus, as the charging/discharging cycle is repeated, pulverization of the active materials, exfoliation of the active materials from current collectors, or the like occurs. As described above, the active materials disclosed in Patent Documents 1 and 2 have a problem in that good charging/discharging cycle property cannot be obtained.
Regarding the problem, it has been found that an electrode for a lithium secondary battery formed by depositing an amorphous silicon thin film or a microcrystalline silicon thin film on a current collector such as a copper foil as an active material by a CVD method or a sputtering method exhibits good charging/discharging cycle property (refer to Patent Document 3). This is because the active material thin film tightly adheres to the current collector.
In addition, a method has been found for manufacturing a current collector by disposing a conductive intermediate layer containing polyimide between the layers containing the silicon-based active material, or between the layer containing the silicon-based active material and the metal foil current collector as a binder, and then, in the state in which the conductive intermediate layer is disposed on the metal foil current collector, sintering the deposited body in a non-oxidizing atmosphere (refer to Patent Document 4). Here, the conductive intermediate layer prevents a mixture layer from being exfoliated from the current collector by the expansion and constriction of the negative electrode active material accompanying a charge/discharge reaction, and thus, adhesion between the mixture layer and the current collector is enhanced.
However, since the active material layer tightly adheres to the current collector in such an electrode for a lithium secondary battery, there is a problem in that a large stress is applied to the current collector due to a volume fluctuation of the active material thin film accompanying the charge/discharge reaction. Due to the stress, deformation occurs in the current collector, wrinkles are generated, and further, the adhesion between the current collector and the active material is deteriorated. Thus, the battery life is reduced.
To suppress stress generation, there is a demand for a current collector which has a higher strength so that the current collector can resist the stress caused by volume expansion of the active material. As one way to enhance the tensile strength of the current collector, it can be considered that the thickness of the current collector is increased. However, there are disadvantages in that a significant enhancement in the tensile strength of the current collector cannot be expected simply by increasing the thickness of the current collector and also the energy density of the battery is reduced due to an increase in the weight and volume of the battery.
Currently, as metal foils for negative electrode current collectors, a copper foil is mainly used. A representative copper foil for the negative electrode current collector includes a copper foil manufactured by rolling and a copper foil (electrolytic copper foil) manufactured by an electrolysis method. However, with respect to high-strengthening of the current collector using the copper foil, there is a limitation in the use of the electrolytic copper foil. Accordingly, a method for manufacturing a high-strength copper foil by a rolling method has been considered, and it has been proposed that the rolled copper alloy foil be used as the negative electrode current collector (refer to Patent Document 5).
However, as the thickness of the rolled copper foil is reduced, manufacturing cost increases. Therefore, the rolled copper foil is expensive. Therefore, it is possible to obtain a thin and high-strength current collector, but this current collector has a problem in that economic efficiency is deteriorated.
Further, the use of the copper foil as the negative electrode current collector is not an optimal choice from the viewpoint of battery properties. When the lithium ion secondary battery normally works, the potential of the negative electrode is less than 2 V vs. Li in many cases, which is very low. However, when a short circuit or over-discharge occurs in the battery, the potential of the negative electrode is more than 3 V vs. Li in some cases. At such high potential, there is a problem in that the copper is rapidly dissolved and battery properties are deteriorated.
Further, since copper is a metal having a large specific gravity (specific gravity: 8.9), in the case where the copper foil is used as the negative electrode current collector, a weight ratio of the negative electrode current collecting foil occupying the battery is relatively increased and energy density per weight of the battery is prevented from being increased. In addition, there is an economic problem such as high cost in the copper foil. For example, the copper foil is expensive compared to an Al foil used in a positive electrode.
From the above-described circumstance, a negative electrode current collecting foil has been desired which is thin, high in strength, lightweight, economic, and excellent in metal elution resistance in over-discharge, and expectations have been placed on an iron-based foil as the material thereof.
Since the electric resistance of iron is large compared to that of copper, it is difficult to use iron as as the current collector due to the iron's property. However, since a battery structure has been enhanced and battery applications and requested properties have been diversified in recent years, the electric resistance is not always a problem.
The following technique for a battery using an iron foil as the negative electrode current collector may be used. In Patent Document 6, it has been proposed that an electrolytic iron foil having a thickness of 35 μm or less be used as the negative electrode current collector. In addition, it has been also proposed that an electrolytic iron foil plated with Ni be used from the viewpoint of corrosion resistance.
However, it is difficult to increase the efficiency in electrolysis and the electrolytic iron foil is not always economic. In addition, Ni plating of the electrolytic foil is a factor which causes an increase in cost. Further, unless thickness of the Ni plating is formed thick (1 μm or more), when it has been over-discharged, Fe elution is unavoidable.
In Patent Document 7, it has been proposed that a metal foil obtained by depositing iron sesquioxide on a surface of an iron foil or a nickel-plated iron foil be used as a negative electrode current collector. However, even in the metal foil, Fe elution is unavoidable during over-discharging, and further, a side reaction easily occurs at the potential of the negative electrode. As a result, battery efficiency or battery life is easily deteriorated.
In Patent Document 8, a current collector of a ferritic stainless steel foil is disclosed. However, since the electric resistance of the ferritic stainless steel foil is large, particularly, if the thickness of the current collector is reduced, a problem such as heating becomes apparent. In addition, the ferritic stainless steel foil is not economic compared to the copper foil.
Generally, in the field of high-strengthening of steel, a component composition or a heat treatment condition is devised and high-strengthening is achieved using a strengthening mechanism such as solid solution hardening, precipitation strengthening or textural strengthening so that various high-strength steel sheets have been put into use. However, if the high-strength steel sheets of the related art are applied to negative electrode current collectors of secondary batteries, under the influence of an additive component or precipitation form thereof, the electric resistance of the high-strength steel sheets tends to increase compared to that of general steel, and particularly, when the thickness is thin, there is a problem in that the tendency becomes stronger.
With respect to a desired negative electrode current collecting foil of a nonaqueous electrolyte secondary battery in the present invention, particularly, there is a strong demand for a reduction in thickness, and thus, in the high-strength steel of the related art, it is difficult to make strength and electric resistance compatible after a reduction in thickness.
In Patent Document 9, a copper coated steel foil for carrying a negative electrode active material of a lithium ion secondary battery is disclosed. However, the strength of the foil does not satisfy a required level and knowledge concerned with compatibility between the high-strengthening and the electric resistance of the foil is not disclosed. In the technique, since the outermost layer is coated with copper which is soft and has deteriorated heat resisting properties compared to steel, particularly, the strength is easily reduced after heating. Further, since the surface is coated with copper, over-discharge solubility is just the same as that of the copper foil and a remarkable effect of improvement by the disclosed configuration cannot be observed.